The International Organization for Standardization has been developing a standardized code for the representation of video signals for digital storage media. The standard is primarily intended for application to digital storage media supporting a continuous transfer rate up to about 1.5M bits/second, such as compact discs. It is intended for non-interlaced video formats having approximately 288 lines of 352 pixels and picture rates around 30 Hz. The standard is described in the document "International Organization for Standardization", ISO-IEC JT(1/SC2/WG1), Coding of Moving Pictures and Associated Audio, MPEG 90/176 Rev. 2, Dec. 18, 1990, which document is incorporated herein by reference for description of the general code format. The system according to this document will hereinafter be referred to an MPEG.
In the MPEG system, successive video frames are compressed according to one of three types of compression algorithms, intraframe coded (I), predicative coded (P), or bidirectional predictive coded (B). An example of which of successive frames are encoded by respective algorithms is illustrated in FIG. 1B. In FIG. 1B the numbered boxes correspond to respective successive frame intervals. The letters above each box correspond to the encoding type applied to the adjacent frame.
Intraframe coding encodes a frame using information from a single frame such that upon decoding the frame may be reconstructed entirely from one frame of I coded information. Intraframe encoding involves performing a discrete cosine transform (DCT) on the picture data and thereafter differentially encoding (DPCM) the DC coefficients generated and variable length coding (VLC) the differentially encoded DC coefficients and the AC coefficients.
Predictive coding involves generating a motion compensated prediction from an immediately preceding I or P frame, that is forward prediction. In this mode translation or motion vectors (MV) are generated which describe the displacement of image areas of the previous I or P frame to similar image areas of the current P frame. A predicted frame is generated using the motion vectors and the video information from the prior I or P frame. The predicted frame is then subtracted from the current frame and the differences (on a pixel basis), termed residues, are successively DCT and VLC encoded. The coded residues and the motion vector constitute the code data for the P frames.
The bidirectionally predictive coded frames occur between I and P or P and P or I and I frames and are encoded similarly to the P frames except that for each frame, motion vectors are generated relative to a successive I or P frame and a prior I or P frame. These motion vectors are analyzed for the best match and the predicted frame is generated from the vector indicated to more accurately predict an image area, or from a weighted average of predicted images using both the forward and backward vectors. Thereafter residues are generated, DCT transformed, and VLC coded. The coded residues and the motion vectors constitute the code data for the B frames.
Luminance, Y, and chrominance U and V, information are encoded separately, however the luminance motion vectors are used for developing both the luminance and chrominance B and P encoded frames. The motion vectors are transmitted only with the luminance information.
At the encoder and decoder ends of the system, the frames, B, that are to be bidirectionally encoded/decoded occur prior to successive P or I frames needed to perform the bidirectional encoding/decoding. Therefore, the sequence of naturally occurring frames are reordered to facilitate encoding/decoding. The reordering is illustrated in FIG. 1C and may be accomplished by simply writing the successively occurring frames to a buffer memory of appropriate capacity and reading the frames from the memory in the desired order. The encoded frames are transmitted in the reordered sequence obviating reordering at the decoder.
Apparatus for selectively performing the three types of compression is known, and described in, for example, the paper "A Chip Set Core for Image Compression" by Alain Artiere and Oswald Colavin, and available from SGS-Thomson Microelectronics, Image Processing Business Unit, 17, avenue des Martyrs-B. P. 217, Grenoble, France, which paper is incorporated herein by reference. This apparatus may be utilized to perform MPEG encoding by appropriate timing to select the compression type for respective frames and adding storage and multiplexing apparatus to add appropriate header information to the compressed data stream.
The MPEG standard transmits 240 lines (NTSC) per frame non-interlaced, which is typically accomplished by encoding only the odd or even fields of an interlaced source video signal, or by subsampling a non-interlaced source signal. In either event this format will not support reproduction of an HDTV image. In addition, since the MPEG standard is primarily directed to computer type display of video images and is expected to be communicated over dedicated transmission lines bit error generation is substantially non existent because the transmission channels are relatively noise free. Conversely, if an MPEG type encoded signal is to be employed for terrestrial HDTV transmission, significant data errors or signal corruption may be expected. As such special techniques are required to provide acceptable image reproduction.